Overheating Issues
A new radiator will only solve part of the problem if you have other issues in the cooling system. Belt slippage is a sneaky overheating gremlin because it isn't always apparent. Make sure your belts are solid and tight.
Another overheat issue is a lower radiator hose that collapses on the open highway. As the engine demands more coolant flow at higher revs, the lower hose tends to collapse from the heavy rush of coolant flow into the block. Make sure you have an anti-collapse coil spring in the lower hose.
Removing the thermostat to solve an overheating problem is not only foolish, it's stupid. Do not remove the thermostat. Run a 180-degree thermostat and check it for proper operation. Inspect coolant flow through the radiator when the engine reaches operating temperature. Use a 7-pound cap on your classic Mustang, not a 12-14-pound cap.
Coolant recovery systems are a great idea because they allow room for coolant expansion when hot, then release coolant back to the engine when cool. If originality is important to you, hide the reservoir in a location you can reach.
Cooling Additives
There are plenty of additives advertised to eliminate cooling-system problems. The only additives you should use are water wetters, which improve the cooling capacity of your coolant mix. Never use stop-leak additives. They may stop leaks, but they can also clog your cooling system and create hot spots in the engine.
Thoroughly flush and service your cooling system every two years regardless of how often you drive the car. Antifreeze has a service life of about two years at the most. After that, the corrosion inhibitors start to degrade. What's more, heat and combustion byproducts get into the coolant and create acids and other contaminants. Flush and fill every two years.
Evans non-aqueous coolant is expensive, but it never has to be replaced. Once installed, you never have to sweat cooling-system service again except to replace hoses every 4-5 years.